Electrical flow-measuring probe

ABSTRACT

A hot-film measuring probe for measuring the shedding frequency of vortices in a medium in which a sensing member is arranged on the vortice generating body. The sensing member or members are preferably arranged to face the direction of flow. A vane on a pivotably mounted body may ascertain correct position of sensing members with changing directions of flow. Practical measurements can be carried out in the higher range of flow in which Reynolds number exceeds 300. A measuring device with a measuring probe as described renders signals which can be converted in digital signals to be counted or stored. A measuring device with a measuring probe comprising two sensing members increases the quality of the combined signal. A measuring device with two probes renders a difference signal indicating the direction of flow.

ilnite Rasmussen Feb. 29, 1972 [54] ELECTRICAL FLOW-MEASURING OTHERPUBLICATIONS PROBE Mair, W. A. The Effect of a Rear Mounted Disc on theDrag [72] inventor; Carl Gem-g Rasmussen, Banal-up, of a Blunt- BasedBody of Revolution. From the Aeronautical Denmark Quarterly. Nov. 1965.pp. 350- 359.

[73] Assignee: Disa Elektronili A/S (Dansk lndustri Syn- PrimaryExaminer-Jerry W. Myracle dikat A/S), Herlev, Denmark AttorneyBeveridge& De Grandi [21] PP N05 862,606 A hot-film measuring probe for measuringthe shedding frequency of vortices in a medium in which a sensing memberis arranged on the vortice generating body. The sensing [3O] FomgnApphcamn Pnomy Data member or members are preferably arranged to facethe Sept. 23, 1968 Denmark ..4569/68 ti n of flow- A van n a pivotablymounted body may ascertain correct position of sensing members withchanging 52 us. Cl ..73/189, 73/204 difectims Practical measurements canbe carried [51] Int. Cl. ..G01p 13/00 in the higher range of flow inwhich Reynolds number exceeds [58] Field of Search ..73/l89, 204, 194 B,194C A measuring device with a measuring probe as described [56]References Cited renders signals which can be converted in digitalsignals to be counted or stored.

UNITED STATES PATENTS A measuring device with a measuring probecomprising two 2,870,305 1/1959 Ling ..73/204 X sensing membersincreases the quality of the combined signal. 3251225 5/1926 f g; Ameasuring device with two probes renders a difference 3,352,] 1 9 7DJOl'Llp Signal indicating the direcfion of flow 3,435,676 4/1969Bruckner ...73/204 X 3,498,127. 3/1970 Richards ..73/204 14 Claims, 6Drawing Figures J L It. 5 6

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INVENTOR amuse-one RASMusst-N ma mama- ATTORNEYS ELECTRICALFLOW-MEASURING PROBE SUMMARY OF THE INVENTION This invention relates toa measuring probe comprising an electrically conductive sensing memberprovided with leads and carried by an insulating carrier member.

Measuring probes of this kind are used for carrying out measurements inflowing media. Many different designs are known, of which somerepresentative ones are described in the specifications to U.S. Pats.Nos. 2,870,305 and 3,333,470.

In an anemometer, in which such a measuring probe is employed,electrical signals are produced by the electrically conductive sensingmember, which is being kept heated by means of an electric current,being cooled by the medium flowing past, so that changes in thetemperature of the sensing member and consequently corresponding changesin the electrical resistance occur.

The measurements can be effected either by keeping the current passingthrough the electrical sensing member constant, or, by keepingthetemperature of the electrical sensing member constant and changing thecurrent concurrent with the variations in the heat emission.

An object of the present invention is to provide a practicallyapplicable measuring device that is capable of generating an electricdigital signal which, within a suitable temperature range, withappropriate accuracy is correlated to the quantity to be measured, e.g.,velocity or volume of a passing medium.

It is known that whereas the flow of liquid around the front of astationary body can, with good approximation, be regarded as a potentialflow, the flow around the rear of the body differs herefrom, in that inthe (boundary) layer vortices are shed which are carried away with theflow. This vortex shedding is of a fairly regular nature when the flowtakes place across a cylinder which is of such a length that one canignore the influence of the ends and can regard the flow as beingidentical in planes at right angles to the axis of the cylinder. In thiscase, vortices are shed alternately from the one and the other side ofthe cylinder, so that the wake comes to comprise a double row ofvortices in a so-called vortex street, in which each vortex liesopposite to the center between two successive vortices in the oppositerow. In view of the great importance flow conditions in fluids have formaritime and aeronautical designs, many studies have been carried out inorder to clarify the conditions; by way of example reference is made toreport No. 1191 of 1954 by the National Advisory Committee forAeronautics by Anatol Roshko. The measurements are effected by means ofa hot wire anemometer placed behind a cylindrical body in a flowingfluid. Many research workers have investigated into the connectionbetween vortex shedding frequencies and fluid-velocity, and in thisconnection numerous measurements of Strouhals number S as a function ofReynolds number R have been made- Strouhals number is determined byS=n,(d/ V,,), in which n is the unilateral vortex shedding frequency,dis the diameter of the cylinder and U,, the free flow velocity.

The results of the measurements show that only for values of Reynoldsnumber between 40 and 150 a stable, regular vortex street is obtained.

At values of Reynolds number between 150 and 300, there existconsiderable instabilities, which increase with rising Reynolds numberand which render the determination of the frequency difficult.

At values of Reynolds number above 300, it is possible to determine thefrequency again by oscillographical methods, but the range is termedirregular on account of the strong noise signals. Incidentally,Strouhals number S is, to all intents and purposes, independent ofReynolds number in this range.

As a-result of the measurements carried out earlier, it has been foundthat it is possible to effect a determination of the fluid-velocity onthe basis of a determination of the vortex shedding frequency withinthat range in which Strouhals number is a function of Reynolds number,i.e., in the stable range for values ofReynolds number between 40 andISO.

The present invention is based on the recognition of the fact that,irrespective of the not very promising results of studies of the rangein which Reynolds number is higher than 300, it is possible to producesignals sufficiently undisturbed for obtaining representative andsufficiently accurate measurements for the practical measuring of, forinstance, flow velocity and flow volume.

The study on which the invention is based has shown that the vortexfrequency signal desired should not be looked for somewhere in the wakeof the cylinder, but on the contrary, on the surface of the cylinderproper.

In the light of this new recognition, the subject matter of theinvention is primarily a new measuring probe which differs from hithertoknown measuring arrangements measuring vortex frequency in flowingfluids, in that the probe combines the body giving rise to the vorticesand the electrical sensor generating the electrical measuring signals.

In accordance with this, the carrier member of the measuring probeaccording to the invention is shaped as a vortexforming member in aflowing fluid, and the sensing member is arranged on a part of thesurface of the carrier member at a distance from the ends of the carriermember. The sensing member extends across such a small area of thesurface of the carrier member, both in its longitudinal and in itscircumferential direction, that the sensing zone can be adjusted inrelation to the place where the vortex shedding occurs.

In an expedient embodiment of the measuring probe according to theinvention, two electrically conductive sensing members with leads forderiving two electric signals, one from each sensing member, arearranged on the surface of the carrier member. This provides thepossibility of obtaining a still better signal by combining the twosignals. With a view to compensation of noise signals, the two sensingmembers can be arranged spaced from each other along the circumferenceof the carrier member in the same cross sections.

The investigations which form the basis of the invention, have led toanother novel recognition, viz that the quality of the signal is,contrary to expectations, improved when the sensing member does not, asshould be expected, face rearward, but, to the contrary, when it facesforward in the direction of flow, namely in such a way that it is turnedat a small angle in relation to the direction of flow.

In accordance herewith, the invention also relates to a measuring devicewith a measuring probe as dealt with here, fitted in a duct for aflowing medium with its axial direction inclined to the direction offlow in which measuring apparatus the measuring probe is mounted withthe sensing member facing the flow direction. If a measuring probe withtwo sensing members is employed, it is preferably disposed in such a waythat there is only one sensing member at each side of a plane parallelto the flow direction and containing the axis of the measuring probe,preferably at an angular distance of l5to 30calculated along thecircumference of the carrier member.

The measuring probe can be mounted pivotably around its own axis andsaid axis can be provided with a vane, which ensures that the measuringprobe maintains the correct position in a freely flowing fluid.

The measuring probe and the measuring device according to the inventionopen the way for quite new possibilities for the carrying out andregistering of measurements as the consequence of the production of a,for practical application, sufficiently interference-free and accuratesignal. In an embodiment of a measuring device according to theinvention the sensing members are through leads connected to anamplifier having a frequency response suitable for the measuring rangedesired, the output of which amplifier can be connected to an electroniccounter for digital indication of velocity or flow.

By combining the counter with a data registration machine, pricecalculations of quantities of liquid delivered can be obtained. In thefollowing, the invention is explained in greater detail by way ofexamples while reference is made to the purely diagrammaticalaccompanying drawings, in which FIG. 1 shows a measuring arrangementwith a measuring probe according to the invention,

tum:

FIG. 2 shows a section through a part of the nozzle like duct shown inFIG. I at right angles to the measuring probe and parallel to thedirection of flow,

FIG. 3 is an idealized curve, which shows the connection betweenReynolds numbers and Strouhals numbers,

FIG. 4 shows a part of an embodiment of a measuring probe according tothe invention with a sensor,

FIG. 5 shows a part of another embodiment of a measuring probe accordingto the invention having two sensors, and

FIG. 6 shows a measuring probe with a vane according to the invention.

The measuring arrangement shown in FIG. 1 is meant to illustrate thefeatures characteristic for the present invention, as well as toindicate an example of a practical application. In FIG. 1, 1 denotes astorage tank for a liquid which, via a line 2, is connected to ameasuring nozzlelike duct 3, the outlet of which is connected with avalve 5, from which the liquid can be supplied to a consumer 6. Thepressure necessary for bringing about a flow of the liquid through theduct can be obtained by the tank 1 being situated at a higher level thanthe duct 3, or by means ofa pump.

The valve 5 can be a solenoid valve, which is opened by a pulse from astarter system which, in the block diagram shown, is denoted start.

In the duct 3, a measuring probe 12 of the hot-film type is inserted,but designed in accordance with the present invention, in a way as willbe explained later.

The electric signal that is produced resulting from the flow of theliquid through the duct, is led from the measuring probe throughelectrical leads 13 to an apparatus of the anemometer type, known perse, and which in the block diagram shown is denoted Anemometer. Theoutput signal from the anemometer is led to a selective AC amplifier andthe pure sine output signal is supplied to a counter and possibly to adata registration apparatus.

The nozzlelike duct 3 is designed with a conical inlet part 14, in whichthe flow velocity of the liquid diminishes in the direction of flow.Thereupon follows a cylindrical part 15, which contains filters andpossibly flow-smoothing members.

The fluid flowing past the measuring probe 12, produces, as illustratedin FIG. 2, a vortex street behind the probe when the flow velocity issufficiently high.

The measurements in the vortex street behind a cylindrical body quotedin the literature, is carried out at some distance from the body bymeans of a sensor that is connected with an anemometer device by meansof which a thin wire or a film on the sensor is kept at a constanttemperature above the temperature ofthe fluid flowing past.

The result of the studies reported in the literature is expressed in theform of curves, in that Strouhals numbers have been depicted as afunction of Reynolds numbers.

In FIG. 3, a greatly idealized rendition of this curve is shown.

Strouhals number is defined by the following expression:

S=n 'd/V in which n is the unilateral vortex shedding frequency, d isthe diameter of the cylinder and V, the free flow velocity.

The range in which Reynolds number lies between 40 and 150 is termed thestable range, whereas the range in which Reynolds number lies betweenI50 and 300, is called the transitional range, which is characterized bygreat instability. Finally, the range in which Reynolds number liesbetween 300 and 10,000, is termed the irregular range since there existsno reproducible unequivocal correlation.

In the first-mentioned range, Strouhals number strongly depends uponReynolds number, whereas in the last-mentioned range it is, in the main,constant and thus independent of Reynolds number.

While in the literature the range, in which Reynolds number lies between40 and 150, is regarded as well-suited for determination of flowvelocities from frequency measurements, this does not apply to the othertwo ranges. In the irregular range it is, admittedly, possible toidentify a frequency by statistical means, the signals have, however,not proved themselves suited for practical application.

The present invention is based on the recognition, that it is the mannerin which the signals have been derived, which has been a principal causefor it being impossible to obtain a usable signal.

Instead of employing a circular-cylindrical body for the production ofthe vortex street in the wake as well as a separate sensor memberdisposed in this vortex street, the inventor has solved the problem ofobtaining a usable signal by combining the vortex-generating body andthe sensor into a unit, so that the sensor is arranged directly on thebody.

Already by this measure a significantly better signal is obtained thanhas hitherto been possible.

By the rotation of the sensor around its axis, it has surprisingly beenfound that the best signals are obtained when the sensor is on that sideof the measuring probe which faces the flowing fluid, while on mighthave immediately expected that the best result would be obtained in thesensing member were to lie on the transition point to the rear of themeasuring probe, where the vortex shedding takes place.

The measuring probe according to the present invention renders signalsin which a component having a frequency which is dependent upon the flowvelocity is clearly distinguishable, and moreover a component which,with great certainty shows zero passages which it is possible to countby means of an appropriate apparatus. By this means it becomes feasibleto convert an analogous signal into a digital signal, whereby apractical utilization of the signal becomes possible.

From Reynolds number:

in which R is Reynolds number, U is the flow velocity, d is the diameterof the vortex cylinder, D is the diameter of the duct and v is thekinematic friction coefflcient, the dimensions can be calculated.

If it is desired, for instance, to measure a volume flow Q of between 6liters per minute and 60 liters per minute and the minimum value forReynolds number is selected to lie at 600, i.e., a suitable distanceabove the transitional range, and there is applied a fluid for which :1equals l.5=l0 cm. /sec., in which case the ratio is found to be D/d=14.2 cm. If, for practical reasons, the diameter of measuring probe isselected to be 0.2 cm., the diameter ofthe duct becomes 1.68 cm.

FIG. 4 shows a part of an embodiment ofa measuring probe according tothe invention. The measuring probe comprises a body 27, which at leaston the part of surface carrying the electrically conductive members iselectrically insulating, but which can consist entirely of glass orquartz or other ceramic material. On the insulating body there is, e.g.,by sputtering, applied a film of conductive material. For use inconductive fluids, the conductive material is covered by a thin film ofinsulating material 17 which, in FIG. 4 is only shown on a small part atthe bottom to the left. The film 17 can be produced by sputtering.Sputtering is a technique known per se, whereby material is atomized byion bombardement and is transferred in an electrical way to form acoating. The carrier body 27 for the electrical film servessimultaneously as vortex cylinder. At the one end shown it is taperedwith a cone 18, by means of which it can be mounted pivotably in abearing, not shown. In order to maintain the carrier body 27 in correctposition under the influence of the flowing fluid a vane 20 can bemounted on the carrier body as is shown in FIG. 6 in which the directionof flow is indicated by an arrow. The carrier body 27 is tapered at bothends and mounted pivotably in bearings 28 carried by a bifurcatedsupport 29 mounted in the measuring duct, not shown. The vane 20 is bymeans of a pair of thin supports 30 connected with the carrier body 27.

It has been found that excellent results are obtained, if the sensingmember is turned from l5to 30from the plane which llll l "ll/h 5contains the direction of flow and the axis of the measuring probe. Thesensing member proper is denoted with 21 in FIG. 4 and is constituted ofa narrow portion of the film 16, the remaining parts of which representthe electric supply leads.

In the embodiment shown in FIG. 5, two sensing members 22 and 23 arearranged, which have a common lead 24 as well as separate leads, 25 and26, respectively. The sensing mem bers can possibly have completelyseparate leads. The main direction of the sensing member lies in thedirection of the generatrix of the carrier body.

The invention is described on the basis of a few examples, but manyother embodiments and uses are feasible. The measuring probe can havemore than two measuring members and the carrier member does not have tobe circular-cylindrical. In the duct, the measuring probe can be mountedinclined in relation to the direction of flow, i.e., at an angle to samediffering from 90, the inclination, however, being sufficiently small toobtain a reasonable large signal-to-noise ratio. By such mounting, acertain automatic cleaning of the measuring probe can be achieved.

The nozzle like duct can have arbitrarily suitable forms of crosssection, even though the circular one will generally be employed forpractical reasons. The dimensioning is selected in such a way thatReynolds number exceeds 500 within the measuring range selected.

The inaccuracies arising from starting and stopping which, as aconsequence of the flow velocity not being sufficiently great, are goingto have a relatively greater influence on the accuracy of the measuringresults when fluids are delivered in small quantities, and provisionought therefore be made for as fast an opening and closing as possible.

Any quantity correlated with the vortex shedding frequency, such asvelocity of flow and volume of flow, can be measured by means of theapparatus according to the invention and the measuring results, whichare available preferably in the form of digital signals, can be employedto control various functions.

The generation of an analogous signal for the direction of flow by meansof the so-called X-hot-wire technique is known.

The measuring probe according to the invention renders the generation ofa digital signal for the direction of flow possible. To this end, twomeasuring probes are used which are disposed in the flowing fluid sothat mutually they form an angle, and are both inclined in relation tothe direction of flow. It has been found that the frequency of thesignals from each of the two measuring probes is dependent of the anglethe axis of the measuring probe forms to the direction of flow. If themeasuring probes are mounted in such a manner that a deviation of thedirection of flow from a given direction, the reference direction, inthe one measuring probe results in a frequency increase and in the otherin a frequency reduction, the frequency difference can be used asmeasure for the angular deviation of the direction of flow from thereference direction.

What I claim is:

1. A flow-measuring probe comprising, a carrier member formed as thesolid circumscribed by translating a generatrix completely around anaxis parallel thereto along a curved path to provide a vortex-generatinglinear body placed in a fluid passing in a direction transverse thereto,

an electrically insulating surface on said body,

a laminar conductive layer superimposed on a leading edge portion ofsaid surface and energized as a resistance element,

and means for sensing the resistance of said element saidlayer'comprising at least a pair of conductive leads extendinglongitudinally of the body being electrically connected on one surfacethereof and having a sensing portion of restricted dimensions relativeto said leads such that current passed therethrough heats said sensingportion to a greater degree and exhibits greater resistive change due tofluid flow therepast than said leads.

2. A probe according to claim 1, said sensing device comprising at leastthree lead portions formed in said conductive layer and at least tworestricted portions each comprising a heating and resistive-changeexhibiting portion of the probe.

3. A probe according to claim 2, said body being a circular cylinder andsaid sensing portions being arranged in spaced relation along thecylinder at less than separation therearound.

4. A probe according to claim 3 said sensing portions being arrangedsymmetrically about a plane passing through said axis and a direction offlow in a surrounding fluid.

5. A probe according to claim 3, said sensing portions being arranged atsubstantially 15 to 30 on either side of a cylinder diameterperpendicular to the direction of flow.

6. A probe according to claim 1, said body being pivotally supportedtransversely of a direction of flow in a surrounding fluid and vanemeans attached to said body for orienting said body with a leading facecontinuously directed upstream.

7. A probe according to claim 1, said layer being externallyelectrically insulated.

8. A measuring device for determining the frequency of shedding ofvortices from either side of a cylindrical body disposed across the flowof a fluid wherein the vortices may be formed continually, comprising agenatrix-formed cylinder means mounting said cylinder for pivotalrotation about the cylinder axis, vane means arranged downstream fromsaid cylinder and connected to continuously orient said cylinderrelative to fluid flow, a pair of electrically conductive stripsarranged symmetrically about the cylinder relative to said vane means,and means for energizing said strips and detecting the change in currenttherein according to fluid velocity therepast.

9. A device according to claim 8, said strips being connected to an ACamplifier to provide amplification of AC components of signalrepresenting said changes in current.

10. A measuring device according to claim 9 including means comparingfrequencies of current variations in the two sensing strips as anindication of direction of flow.

11. A device according to claim 8, said cylinder being mounted in afluid duct wherein the Reynolds number exceeds about 500 and said stripsare both arranged along the cylinder facing the fluid flow.

12. A measuring device comprising a probe according to claim 8, an ACamplifier connected to a pair of leads to said sensing portion foramplification of periodic variations in resistance thereof, and meansindicating the frequency of said variations as a digital count.

13. The method of measuring fluid flow along a pathway which comprisesobstructing a portion of said pathway with a cylindrical solid objectdisposed transversely thereto to generate a vortex sheet downstream ofthe object, and

detecting on at least one upstream shoulder as resistive changes in aconductor an AC signal corresponding in frequency to variations in rateof fluid flow said AC signal frequency corresponding to the frequency ofvortex shedding in said sheet.

14. A flow-measuring probe comprising,

a carrier member formed as the solid cylinder circumscribed bytranslating a generatrix completely around an axis parallel theretoalong a curved path to provide a vortex generating linear body whenplaced transversely of a moving fluid,

an electrically insulating surface on said body extending between endportions thereof,

a laminar electrically conductive layer superimposed on portions of saidsurface between said end portions,

said layer comprising at least a pair of conductive leads extendinglongitudinally of the body and a sensing portion of restricted lateraldimension relative to the lateral dimension of said leads such that thesensing portion exhibits greater resistive changes with temperature thansaid leads,

means energizing said sensing portions by way of said leads to comprisea variable resistance element, and means for sensing said variableresistance.

1. A flow-measuring probe comprising, a carrier member formed as thesolid circumscribed by translating a generatrix completely around anaxis parallel thereto along a curved path to provide a vortex-generatinglinear body placed in a fluid passing in a direction transverse thereto,an electrically insulating surface on said body, a laminar conductivelayer superimposed on a leading edge portion of said surface andenergized as a resistance element, and means for sensing the resistanceof said element said layer comprising at least a pair of conductiveleads extending longitudinally of the body being electrically connectedon one surface thereof and having a sensing portion of restricteddimensions relative to said leads such that current passed therethroughheats said sensing portion to a greater degree and exhibits greaterresistive change due to fluid flow therepast than said leads.
 2. A probeaccording to claim 1, said sensing device comprising at least three leadportions formed in said conductive layer and at least two restrictedportions each comprising a heating and resistive-change exhibitingportion of the probe.
 3. A probe according to claim 2, said body being acircular cylinder and said sensing portions being arranged in spacedrelation along the cylinder at less than 180* separation therearound. 4.A probe according to claim 3 said sensing portions being arrangedsymmetrically about a plane passing through said axis and a direction offlow in a surrounding fluid.
 5. A probe according to claim 3, saidsensing portions being arranged at substantially 15* to 30* on eitherside of a cylinder diameter perpendicular to the direction of flow.
 6. Aprobe according to claim 1, said body being pivotally supportedtransversely of a direction of flow in a surrounding fluid and vanemeans attached to said body for orienting said body with a leading facecontinuously directed upstream.
 7. A probe according to claim 1, saidlayer being externally electrically insulated.
 8. A measuring device fordetermining the frequency of shedding of vortices from either side of acylindrical body disposed across the flow of a fluid wherein thevortices may be formed continually, comprising a genatrix-formedcylinder means mounting said cylinder for pivotal rotAtion about thecylinder axis, vane means arranged downstream from said cylinder andconnected to continuously orient said cylinder relative to fluid flow, apair of electrically conductive strips arranged symmetrically about thecylinder relative to said vane means, and means for energizing saidstrips and detecting the change in current therein according to fluidvelocity therepast.
 9. A device according to claim 8, said strips beingconnected to an AC amplifier to provide amplification of AC componentsof signal representing said changes in current.
 10. A measuring deviceaccording to claim 9 including means comparing frequencies of currentvariations in the two sensing strips as an indication of direction offlow.
 11. A device according to claim 8, said cylinder being mounted ina fluid duct wherein the Reynolds number exceeds about 500 and saidstrips are both arranged along the cylinder facing the fluid flow.
 12. Ameasuring device comprising a probe according to claim 8, an ACamplifier connected to a pair of leads to said sensing portion foramplification of periodic variations in resistance thereof, and meansindicating the frequency of said variations as a digital count.
 13. Themethod of measuring fluid flow along a pathway which comprisesobstructing a portion of said pathway with a cylindrical solid objectdisposed transversely thereto to generate a vortex sheet downstream ofthe object, and detecting on at least one upstream shoulder as resistivechanges in a conductor an AC signal corresponding in frequency tovariations in rate of fluid flow said AC signal frequency correspondingto the frequency of vortex shedding in said sheet.
 14. A flow-measuringprobe comprising, a carrier member formed as the solid cylindercircumscribed by translating a generatrix completely around an axisparallel thereto along a curved path to provide a vortex generatinglinear body when placed transversely of a moving fluid, an electricallyinsulating surface on said body extending between end portions thereof,a laminar electrically conductive layer superimposed on portions of saidsurface between said end portions, said layer comprising at least a pairof conductive leads extending longitudinally of the body and a sensingportion of restricted lateral dimension relative to the lateraldimension of said leads such that the sensing portion exhibits greaterresistive changes with temperature than said leads, means energizingsaid sensing portions by way of said leads to comprise a variableresistance element, and means for sensing said variable resistance.